The present invention relates to the sealing of lidding stock to the flange of an open ended container in a manner that permits peeling of the stock from the flange. (Lidding stock is generally a multi-layered sheet of material provided with a sealing coating, layer or film.) More particularly, the invention is directed to a head or plate structure for heat sealing lidding stock to a container flange.
Lidding stock is sealed to a container flange by a plate or ring means that applies heat and contact pressure to the lidding stock and flange. The plate or ring, in plan view, has a peripheral face of the general shape and size of the flange so that when the face is forced against the stock the sealing layer of the stock will soften the entire distance around the flange and adhere to the flange surface to form an effective seal between the stock and flange. The peripheral, sealing face of the plate or ring extends in a direction perpendicular to the plane of the plate or ring.
In sealing lidding material to the flange of a container, it is desirable that the head or plate structure that effects the seal soften and indent the flange to form a more effective seal. However, head structures presently in use and designed to effect indentation of the container flange employ an indentor that forces the sealing layer of the lidding stock outwardly from the sealing area beneath the indentor and, similarly moves the softened (molten) flange material from the sealing area, as explained in detail hereinafter, in effect cutting through the sealing film, as it is pressed against the container flange. When the consumer attempts to peel back the lid from the flange, the sealing material of the stock tends to remain intact on the container, at least in part, while the lid is removed. The reason for this is explained below. The manufacturers and suppliers of packaged items using lidding stock require the entire seal to be removable from the flange.
As explained hereinafter, the problem with heat sealing techniques currently in use is that they do not care for the rheology of the heat sealing and container material and the non-uniformity of heat transfer involved in the sealing process. (Rheology refers to the manner in which materials deform.)